Each engine OEM has a different way of doing it, and it also depends on the engine model. All you have to remember is the outgoing water from the engine to the heater should be tapped into a port on the pressure side after the circ pump, usually a port on the circ pump, the t-stat housing on a Merc, or intake manifold, and the return line from the heater should be tapped into the suction side of or before the circ pump like the large heater hose. Merc sells a Tee fitting for that hose just for this purpose.
Unfortunately the heater manual will only give you a general idea like I just did above as there are too many engine models to cover.
Do you not have a hot water heater on board with a heat exchanger in it that runs off the engine?? If you do, it hooks up the same way. Just copy that on the other engine for the space heater.
Go half way down the page on this link and you'll see an example. The fitting on the large hose is not the Merc Tee I mentioned but it gives you an idea. The line on the left into the big hose is the return, and the one on the right with red tracer is the output or pressure side....
https://coxengineering.sharepoint.com/Pages/Calorifier.aspx